Is your doctor squeamish about testosterone replacement? Many are; in fact, to many doctors, the idea of testosterone replacement has ranked right up there with playing with fire. They worry that men will get burned – that increasing a man’s testosterone will make him more susceptible to prostate cancer. They worry about this because the mainstay of treatment for advanced prostate cancer is to do just the opposite – to shut down testosterone, and cut off the supply of male hormones to cancer cells.

But there’s good and surprising news: Boosting low testosterone doesn’t seem to raise the risk. In fact, it significantly reduces a man’s risk of getting aggressive prostate cancer.

Before we get carried away and say, “Extra testosterone for every man!” and run around tossing out gel patches, pills, and shots, we need to talk about what this actually means.

Let’s look at the study, recently published in the Journal of Clinical Oncology. In an international effort, scientists from New York University and Sweden analyzed the health records of more than 250,000 men on several health care registries in Sweden, including the National Prostate Cancer Register. There were 38,570 men who developed prostate cancer between 2009 and 2012, and 284 of them were on testosterone replacement therapy (TRT) before they were diagnosed. The investigators compared those men with 192,838 men who did not have prostate cancer, 1,378 of whom were on TRT.

What they found completely up-ended their hypothesis that giving replacement testosterone might be risky. “We found no relationship between the use of testosterone and the development of prostate cancer as a whole,” urologist Stacy Loeb, M.D., the study’s first author, told me when I interviewed her for the Prostate Cancer Foundation’s website, pcf.org. Even better, “we found that long-term use (more than a year) of TRT is associated with a much lower risk of aggressive cancer – a 50 percent lower risk.”

Testosterone actually lowers prostate cancer risk? Well, it does for the men who don’t have enough of it to start with. The men in Sweden who were on TRT, the investigators believe, actually need to be on it, and are prescribed it by their doctor. That’s not always the case in the U.S., where it’s common to see TV ads telling men that if they’re tired and have a low sex drive, they may have “Low T,” and offering prescription help.

So what we’re talking about here are men with below-normal testosterone who take medicine to get their testosterone level back up to normal range. “There are some interesting tie-ins to this,” says Loeb. Previous studies have shown higher rates of high-grade prostate cancer in men who don’t produce enough testosterone, “so it’s definitely a possibility that restoring the testosterone to the normal range could prevent this from happening. ”

In the study, researchers noticed an initial bump of men diagnosed with prostate cancer soon after they started TRT. However, these were low-risk cancers (easily treatable; in fact, many low-risk cancers can safely be treated with active surveillance), and Loeb believes the reason they were diagnosed at all was most likely because the men had received prostate cancer screening when they started the TRT.

How do you know if you have low testosterone? The symptoms, just as the TV ads claim, include fatigue, low libido, and decreasing muscle mass. “But a man should never just start taking a testosterone supplement just because he has those symptoms, because a lot of other diseases can mimic low testosterone,” notes Loeb. And if you have erectile dysfunction (ED): “You’re better off taking a medication such as Viagra, Levitra or Cialis. A lot of men come to the doctor thinking they want to be on testosterone. They’ve seen the direct-to-consumer advertising, and they just want it. But that doesn’t mean it’s the right solution for them.”

However, Loeb adds: “Don’t be afraid to supplement your testosterone if it’s low. There are certainly risks to having a low testosterone level for years; it can affect your cardiovascular system and your musculoskeletal health.” And this new study suggests that raising that testosterone level back up to where it should be may even reduce your risk of getting aggressive prostate cancer.

This research has raised new questions, including:

What about men with low testosterone who have been treated for prostate cancer and are presumably cured? Is it safe for them to go back on testosterone? “Our study did not include men who already had a diagnosis of prostate cancer. These men should be evaluated on a case-by-case basis,” says Loeb.

Why do low levels of testosterone lead to aggressive prostate cancer in some men? And could restoring normal levels of testosterone mitigate this risk? “We have a lot more work to do to understand the implications of these findings.”

In addition to thebook,I have written about this story and much more about prostate cancer on the Prostate Cancer Foundation’s website, pcf.org. The stories I’ve written are under the categories, “Understanding Prostate Cancer,” and “For Patients.” As Patrick Walsh and I have said for years in ourbooks,Knowledge is power: Saving your life may start with you going to the doctor, and knowing the right questions to ask. I hope all men will put prostate cancer on their radar. Get a baseline PSA blood test in your early 40s, and if you are of African descent, or if cancer and/or prostate cancer runs in your family, you need to be screened regularly for the disease. Many doctors don’t do this, so it’s up to you to ask for it.